Field of the Invention
The present invention relates to a press molding apparatus for an optical element for use in forming a high-precision optical element such as an aspherical lens.
Related Background Art
Recently, instead of an optical element forming method with grinding and polishing, a method has been noted in which molding optical glass material heated and softened is directly press molded within a mold.
Normally, in this type of molding, a press molding apparatus is used in which a glass material in a softened state is pressed within a shell mold, using mold members slidable with the shell mold, and an optical functional surface corresponding to a molding surface of each mold member is formed in the glass material. In order to obtain an optical element with a desired degree of precision, it is important to align the center of the optical functional surface of the optical element to be formed with the optical axis of the molding surface of each mold member, in such a manner that the mold members are pressed completely by the time the press is completed and the clearance between the shell mold and each mold member is as small as possible.
In the molding apparatus as above described, for the temperature control of a glass material in the press molding process, it is necessary to heat and cool the shell mold and upper and lower mold members with considerable differences between high and low temperature. For this purpose, the shell mold and the upper and lower mold members are made of materials having substantially the same thermal expansion coefficients, and a clearance is provided to secure the sliding of each mold member with respect to the shell mold. Therefore, for example, when the upper mold member is lowered to press mold the glass material with the lower mold member, the upper mold member may be inclined during sliding within the shell mold, unless the pressure is not applied on the center of the upper mold member, so that the glass material can not be press molded in a state where the upper and lower mold members are correctly aligned with each other. Further, in an extreme case, there occurs some scuffing between the shell mold and the upper mold members, so that the complete closing of the mold members is impossible and thus the normal pressing can not be performed. In other words, as a result, the center of the optical functional surface of a formed optical element is not aligned with the optical axis. When the upper mold member is lifted upon taking out the molded product, the upper mold member may be inclined if an off center lifting force is exerted, thereby causing a scuffing between the shell mold and the upper mold member, so that the mold members can not be opened or closed. Such a molding apparatus has a clearance in a sliding portion between the shell mold and the upper mold members as small as 10 .mu.m or less under practical use condition, and is used in the hot conditions, whereby scuffing is more likely to occur.